This invention relates to transducers to produce signals indicative of the displacement or motion of a mechanically deflecting member. In particular, the invention has use (a) as an accelerometer where the deflection of the deflecting member is caused by an acceleration or (b) as a pressure transducer where deflection is caused by response to a pressure change. The invention further relates to microwave resonant structures in which modifications of normal modes of the electromagnetic microwave field is used to detect motion of a component of the apparatus.
It is well known that the perturbation of the electromagnetic fields contained within a resonant structure will produce a corresponding change in the resonant frequency of the structure's electromagnetic normal modes. U.S. Pat. No. 3,636,752 to Ishii, for example, disclosed a microwave acceleration sensor which employed a microwave cavity resonator having a ferrite rod that was stressed in response to the component of an acceleration along the axis of the rod. Because of the nature of the ferrite, the stresses induced changes in the electromagnetic field within the ferrite due to changes in the electromagnetic permeability of the ferrite. The device was capable of measuring only one component of its acceleration. Apparently, multiple devices would be necessary to determine the complete acceleration vector.
U.S. Pat. No. 3,909,713 to Billetes measured the displacement of a test specimen which functioned as the inner conductor of a coaxial cavity resonator. The test specimen was fixed to a moveable wall of the resonator so that deformation of the specimen varied the dimensions of the resonator cavity. The change in resonant frequency in two different electromagnetic modes were detected, which corresponded to detection of changes in the diameter and axial dimension of the cavity. Vibration transducers have also been fashioned by having one surface of a wave guide cavity undergo the vibration that is to be measured. These devices each required the deformation of the overall dimensions of the cavity in order to function.
The U.S. Pat. No. 3,066,267 to Menhennett is also relevant to prior knowledge of the relationship between the electrical size of a cavity and the frequencies of various electromagnetic modes. The general theory may also be found in standard text such as Jackson, "Classical Electrodynamics" Ch. 8, pp. 235-67, 1962 Ed.
Acceleration measurement using microwave detection of overall cavity dimension variation has also been reported in attempts to detect gravity waves generated by, for example, supernovas. See, for example, Blair, et al., "Microwave Non-Contacting Accelerometer For Gravity Wave Antenna". IEEE Trans. Vol. Mag-13, No. 1, Jan. 1971. An ultra-sensitive superconducting accelerometer has been described where a displacement of a mass along a single axis of motion increases the capacitance in an electromagnetic resonant circuit formed of high Q superconducting resonant cavities. See "Operation Of An Ultrasensitive Superconducting Accelerometer", IEEE Trans. Vol. Mag.-17, No. 1, Jan., 1981, and "Superconducting Accelerometer Using Niobium-On-Sapphire RF Resonator", Rev. Sci. Instrum. 50(3) pp. 286-91 May, 1979.
The foregoing systems are all characterized as capable of providing displacement and acceleration transducers that are limited either by requiring that the resonant cavity itself is deformed by the displacement or acceleration that is to be measured, or that are limited by in general being capable of measuring only one component of the vector displacement or acceleration of interest.